You cold call them. You find people in forums, communities, etc. who have complained about the problem your solving, or are active powerusers in the industry, and you message them. You tell them, "hey, I read in your blog that you were having trouble with X, I'm working on a solution. Can I email you to ask about your experience with [1][2][3]?"

Even better if you can skype interview with them. Best of luck.